1. Field of the Invention
The present invention generally relates to a turbine blade for a gas turbine engine and more specifically to an improved airfoil profile having reduced heat load to the airfoil leading edge resulting in improved life.
2. Description of Related Art
As the demand for more efficient turbine engines continues to increase, higher firing temperatures are required in order to optimize turbine performance. This, in turn, requires enhanced airfoil configurations to accommodate these higher firing temperatures. Known airfoil failure modes, such as creep, which is when an airfoil is exposed to high operating temperatures and a given stress level for an extended period of time, are being addressed through redesigns involving enhanced cooling to reduce airfoil operating temperatures. Further enhancements have also been made to address performance issues caused by boundary layer flow separation. Early turbine blade technology often had airfoils with a blunt or rounded leading edge. A rounded leading edge had a constant radius of curvature, which made for an abrupt transition to the pressure side and suction side of the airfoil body, due to the discontinuous radii of curvature for each surface when compared to the constant radius of curvature of the leading edge. This transition section created regions of rapid acceleration followed by deceleration resulting in performance loss by the turbine blade. To correct this transition, some airfoil designers chose to provide an airfoil having a sharper leading edge, as disclosed in U.S. Pat. No. 5,980,209, and hereby incorporated by reference. The sharper leading edge contained a more elliptical shape that provided a smoother transition to the pressure side and suction side surfaces, thereby reducing the amount of overspeed and improving performance.
While enhancements have typically focused on lowering operating temperatures of the airfoil to increase creep margin and airfoil life as well as to address minor performance issues, there are other failure modes that must be addressed when enhancements are made to an airfoil. One specific area that should be addressed is the xe2x80x9cheat loadxe2x80x9d, of the airfoil leading edge. Heat load is defined as the product of the heat transfer coefficient for a particular airfoil design and the relevant airfoil surface area. While changing the airfoil leading edge to a more elliptical design smooths the transition to the pressure side and suction side surfaces of the airfoil, it has been determined that the heat load experienced by the airfoil leading edge is adversely impacted. Due to the geometry changes, the airfoil leading edge is more difficult to cool than the rounded leading edge configuration of the prior art, resulting in increased heat load. If too large of a heat load is experienced by a specific region of the airfoil, such as the leading edge, it can cause a life limiting condition to be present.
Therefore, what is needed is an airfoil design that incorporates performance and life enhancements of the prior art while minimizing heat load to the leading edge.
In accordance with the present invention, there is provided a novel and improved airfoil having improved performance and reduced operating temperatures for increased creep life, while simultaneously minimizing the amount of heat load experienced by the airfoil leading edge, thereby extending airfoil life. To accomplish this, airfoil geometry is disclosed that contains a semi-elliptical leading edge allowing sufficient cooling to reduce exposure of the leading edge to excessive heat, while maintaining the flow benefits of the transition between an elliptical leading edge and the pressure side and suction side surface curvatures.
In the preferred embodiment of the present invention, an airfoil for a turbine blade having an attachment with a platform extending radially outward from the attachment is disclosed with the airfoil having an uncoated profile substantially in accordance with Cartesian coordinate values of X, Y, and Z as set forth in Table 1, carried only to three decimal places, wherein Z is a distance measured radially from the platform to which the airfoil is mounted.
In an effort to reduce the overall blade heat load, the turbine blade containing the disclosed airfoil geometry contains a reconfigured leading edge, pressure side surface, and suction side surface as well as a plurality of radially extending holes for passing a cooling medium through the airfoil. The cooling medium can vary depending on engine conditions, but is typically compressed air or steam. To protect the airfoil surfaces from oxidation a metallic coating is applied.
It is an object of the present invention to provide a turbine blade having a novel and improved airfoil geometry with improved performance, lower heat load to the airfoil leading edge, enhanced cooling, increased creep margin, and extended life.
In accordance with these and other objects, which will become apparent hereinafter, the instant invention will now be described with particular reference to the accompanying drawings.